1. Field of the Invention
This invention relates to a transparent electrode (positive electrode) and particularly to a transparent electrode possessing excellent perviousness and an ohmic property and fit for use in a gallium nitride-based compound semiconductor light-emitting device which releases an emission in the ultraviolet region.
2. Description of Prior Art
In recent years, the GaN-based compound semiconductor material has been attracting attention as a semiconductor material for use in a short wavelength light-emitting device. The GaN-based compound semiconductor is formed by using a varying oxide substrate starting with sapphire single crystal or a varying Group III-V compound substrate and stacking a relevant compound thereon by the metalorganic chemical vapor deposition method (MOCVD method), molecular beam epitaxy method (MBE method), etc.
The GaN-based compound semiconductor material is characterized by the smallness of current diffusion in the lateral direction. Though the cause for this small current diffusion has not been thoroughly elucidated, it may be logically ascribed to the presence of numerous dislocations occurring in an epitaxial crystal and threading the surface thereof from a substrate. Further, the p-type GaN-based compound semiconductor (occasionally abbreviated hereinafter as a “p layer”) has a high specific resistance as compared with the specific resistance of the n-type GaN-based compound semiconductor. The p-type semiconductor layer, when having a metal simply stacked on the surface thereof, exhibits substantially no lateral diffusion of current therein. When an LED structure is provided with a p-n junction, it emits light only directly below a positive electrode.
In the circumstances, the practice of enhancing the diffusibility of current in the p layer by lowering the specific resistance of the p layer by dint of the radiation of electron beams or the anneal at a high temperature is in vogue. The radiation of electron beams, however, necessitates a very expensive device and does not prove advantageous in terms of cost of production. It further incurs difficulty in imparting a uniform treatment to the whole interior of the wafer. The annealing treatment at a high temperature requires the process to proceed at a temperature exceeding 900° C. for the sake of enabling the effect thereof to be manifested conspicuously. During this process, the crystal structure of the GaN begins to decompose and the possibly ensuing desorption of nitrogen results in degrading the voltage characteristic in the reverse direction.
It has been proposed to promote the decrease of the specific resistance of the p layer and accomplish the formation of a positive electrode endowed with perviousness and an ohmic property by depositing Ni and Au each in an approximate thickness of several tens of nm on the p layer and subjecting the resultant composite positive electrode to an alloying treatment (refer, for example, to Japanese Patent No. 2803742).
In the case of the transparent electrode using Au, however, the use thereof in a device for emitting ultraviolet rays entails the problem that the output of emission is decreased to a great extent. Though Au is a metal that exhibits an excellent transmission factor in the blue color region, it cannot be expected to excel in transmittance because the transmission factor thereof in the ultraviolet region not exceeding 440 nm is about 90% of that in the blue color region.
Then, the alloying treatment in the atmosphere of oxygen entails such problems as giving rise to an oxide layer on the surface of the exposed n-type GaN layer, affecting the ohmic property of the negative electrode, causing the electrode to form a network structure and tending to induce uneven emission.
It has been further proposed to enable the p layer to attain a decrease of the specific resistance and undergo an alloying treatment simultaneously by forming a positive electrode of Pt on the p layer and heat-treating the resultant substance in an atmosphere containing oxygen (refer, for example, to JP-A HEI 11-186605). This method, however, also entails the aforementioned problem because it requires a heat treatment in an atmosphere of oxygen. In order for the simple substance of Pt to form an excellent transparent electrode, the thickness of the electrode must be decreased to a veritably thorough extent (5 nm or less). This necessity results in heightening the electric resistance of the Pt layer, impairing the diffusion of current even when the decrease of the specific resistance of the Pt layer is attained by the heat treatment, depriving the emission of uniformity and inducing a rise of the voltage in the forward direction (VF) and a drop of the intensity of emission.
This invention, with the object of solving the problem mentioned above, is aimed at providing an electrode (positive electrode) exhibiting excellent perviousness in the ultraviolet region, acquiring a low contact resistance and excelling in diffusibility of current without undergoing irradiation with electron beams, an anneal at a high temperature or an alloying heat treatment in an oxygen atmosphere.